Glyocoproteins have been isolated from sheep, cattle and horse erythrocyte membranes which react specifically with heterophile antibody in the sera of patients with infectious mononucleosis. These materials are being characterized chemically, physicochemically and immunochemically with the ultimate goal of complete elucidation of the immunoreactive groups of the molecules and their relationships to the membrane structure and similar structures in membranes of human cells. These purified glycoprotein antigens are being used to isolate and characterize the heterophile antibody in infectious mononucleosis sera. Purified antigens will be used to study responses of lymphocytes from patients with infectious mononucleosis. Purified antigens and antibodies will be used to detect the stimulating antigen, if present, in human cells from patients. Infectious mononucleosis is thought to be remarkably similar to the malignant lymphoproliferative disorders except that it is self-limiting. The proposed studies may give new insight into the relationship of the heterophile antibody response to virus induced altered cell surface antigens and subsequent control of cell proliferation.